Matching the impedance of an RF feed, (antenna) including the resonant cavity, while the resonator optionally includes a load such as food, with the impedance of the RF source increases the efficiency of energy delivery into the resonator and the load. When impedances are perfectly matched no power reflection exists and all transmitted energy is absorbed by the load. As the impedance becomes unmatched, the reflections increase and consequently heating efficiency decreases.
Measuring the reflection from a resonant cavity (return loss) in a microwave oven and modifying the impedance of the feed or the cavity to reduce reflections, or modifying the feed frequency to match the impedance in a microwave oven are described, for example, in U.S. Pat. No. 5,347,108 or U.S. Pat. No. 4,415,789 or J. D. Penfold, et al., Control of Thermal Runaway and Uniformity of Heating in 20 the Electromagnetic Warming of a Cryopreserved Kidney Phantom, in Cryobiology 30,493-508 (1993), the disclosures of which are incorporated herein by reference.